Methods of forming structures having nanotubes extending between opposing electrodes and structures including same

ABSTRACT

A semiconductor structure including nanotubes forming an electrical connection between electrodes is disclosed. The semiconductor structure may include an open volume defined by a lower surface of an electrically insulative material and sidewalls of at least a portion of each of a dielectric material and opposing electrodes. The nanotubes may extend between the opposing electrodes, forming a physical and electrical connection therebetween. The nanotubes may be encapsulated within the open volume in the semiconductor structure. A semiconductor structure including nanotubes forming an electrical connection between source and drain regions is also disclosed. The semiconductor structure may include at least one semiconducting carbon nanotube electrically connected to a source and a drain, a dielectric material disposed over the at least one semiconducting carbon nanotube and a gate dielectric overlying a portion of the dielectric material. Methods of forming the semiconductor structures are also disclosed.

TECHNICAL FIELD

Embodiments of the invention relate to methods of forming structuresand, more specifically, to methods of forming switching elements andtransistor devices including control structures including a nanotubecomponent.

BACKGROUND

Fabricating nano-scale devices that function as ohmic contacts and havelow resistance has been a challenge to the semiconductor industry.Conventional low resistance ohmic contacts are made of metal silicidesformed on heavily doped semiconductor regions. The contact resistance isinversely proportional to contact area. In nano-scale devices, thecontact area is on the order of nanometer or smaller and, thus, contactresistance limits performance.

Carbon nanotube structures are finding their way into nano-scale devicesdue to their unique electron transport properties. The carbon nanotubesmay be metallic or semiconductive. Carbon nanotubes have high currentdensities, such as up to about 10⁹ A/cm². This and other properties ofthe carbon nanotubes make them ideal candidates for use inmolecular-scale electronic devices. Due to their low dimensionality,structural symmetry and electronic properties, carbon nanotubes arebeing explored for use in transistors, interconnects and switches fornon-volatile memory applications.

Integrated circuits combining carbon nanotubes with a siliconmetal-oxide semiconductor field-effect transistor (MOSFET) switchingcircuit have been formed by growing the carbon nanotubes onto theintegrated circuit at predefined locations. Conventionally, the carbonnanotubes are positioned on gate oxides and contacted by polycrystallinethin film metal electrodes or are overgrown epitaxially by high-kdielectrics using atomic layer deposition.

Presently, the manufacturing of carbon nanotube memory devices dependson a so-called “top-down” fabrication technique. For example, a filmincluding a monolayer of nanotubes is deposited on an electricallyconductive material using a spin-on technique and is lithographicallypatterned to make columns and rows of the nanotubes. The 1 nm to 2nm-thick, patterned nanotube is interconnected with complementarymetal-oxide semiconductor (CMOS) circuitry.

Semiconductor structures have also been formed including so-called“ribbons” of carbon nanotubes that are suspended over a carbonsubstrate. In the “off” state, the ribbon of carbon nanotubes does nottouch the carbon substrate, and electricity does not flow between aninterconnect suspending the ribbon. In the “on” state, the carbonnanotubes bend downward and adhere to the carbon substrate through vander Waals forces, enabling electricity to flow between the interconnect.However, bit failure is a common occurrence with the above-mentionedtechniques because reliable contact between electrodes and the carbonnanotubes has not been achieved.

Semiconductor structures with improved control structures are desired,as are methods of forming such semiconductor structures.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIGS. 1-4A are cross-sectional views of embodiments of semiconductorstructures of the invention during various stages of fabrication;

FIGS. 4B and 4C are exploded views illustrating a portion of thesemiconductor structure shown in FIG. 4A;

FIGS. 5-8 are cross-sectional views of embodiments of semiconductorstructures of the invention during various stages of fabrication; and

FIGS. 9-13 are cross-sectional views of another embodiment ofsemiconductor structures of the invention during various stages offabrication.

DETAILED DESCRIPTION

Methods of fabricating structures including nanotubes are disclosed, asare structures including the nanotubes. The structures have improvedelectrical contact between the nanotubes and electrodes relative to thatoffered by conventional nanoscale structures.

In the following detailed description, reference is made to theaccompanying drawings, which form a part hereof, and in which is shown,by way of illustration, specific embodiments in which the invention maybe practiced. These embodiments are described in sufficient detail toenable a person of ordinary skill in the art to practice the invention.However, other embodiments may be utilized, and structural, logical, andelectrical changes may be made without departing from the scope of theinvention. The drawings presented herein are not necessarily drawn toscale and are not actual views of a particular semiconductor structureor fabrication process thereof, but are merely idealized representationsthat are employed to describe the embodiments of the invention.Additionally, elements common between drawings may retain the samenumerical designation.

The following description provides specific details, such as materialtypes, material thicknesses, and processing conditions in order toprovide a thorough description of embodiments of the invention. However,a person of ordinary skill in the art would understand that theembodiments of the invention may be practiced without employing thesespecific details. Indeed, the embodiments of the invention may bepracticed in conjunction with conventional semiconductor fabricationtechniques employed in the industry. In addition, the descriptionprovided below does not form a complete process flow for manufacturing asemiconductor device in which the semiconductor structure is present,and the semiconductor devices described below do not form a completeelectronic device. Only those process acts and semiconductor structuresor semiconductor devices necessary to understand the embodiments of theinvention are described in detail below. Additional processing acts toform a complete semiconductor device from the semiconductor structuresor to form a complete electronic device from the semiconductor devicemay be performed by conventional fabrication techniques, which are notdescribed herein.

The materials described herein may be formed by any suitable techniqueincluding, but not limited to, spin coating, blanket coating, chemicalvapor deposition (“CVD”), atomic layer deposition (“ALD”), plasmaenhanced ALD, or physical vapor deposition (“PVD”). Alternatively, thematerials may be grown in situ. Depending on the specific material to beformed, the technique for depositing or growing the material may beselected by a person of ordinary skill in the art. While the materialsmay be formed as layers, the materials are not limited thereto and maybe formed in other configurations.

FIG. 1 shows a semiconductor structure 10 that includes an electricallyinsulative material 12, an electrode material 14, a dielectric material16, a flowable material 18 and a mask material 20. The electrodematerial 14 may be formed over and in contact with the electricallyinsulative material 12, the dielectric material 16 may be formed overand in contact with the electrode material 14, the flowable material 18may be formed over and in contact with the dielectric material 16, andthe mask material 20 may be formed over and in contact with the flowablematerial 18. The electrically insulative material 12, the electrodematerial 14, the dielectric material 16 and the flowable material 18 maybe formed by conventional techniques, which are not described in detailherein. For example, the electrically insulative material 12 may have athickness of less than or equal to about 500 Å and may be formed from adielectric material such as silicon dioxide (“SiO₂”). In someembodiments, the electrically insulative material 12 may be depositedover a substrate (not shown) by conventional techniques, which are notdescribed in detail herein. As used herein, the term “substrate” meansand includes a base material or construction upon which materials, suchas the electrically insulative material 12 and electrode material 14,are deposited. The substrate may be a semiconductor substrate, a basesemiconductor layer on a supporting structure, a metal electrode or asemiconductor substrate having one or more layers, structures or regionsformed thereon. The substrate may include, for example,silicon-on-insulator (SOI) type substrates, silicon-on-sapphire (SOS)type substrates, and epitaxial layers of silicon supported by a layer ofbase material. Semiconductor type materials may be doped or undoped.

By way of non-limiting example, the electrode material 14 may have athickness in a range of from about 10 Å to about 1000 Å and may beformed from a conductive material, such as a transition metal, aconductive metal oxide, or a metal carbide. More specifically, theelectrode material 14 may include tungsten, hafnium, zirconium,titanium, tantalum, aluminum, ruthenium, palladium, platinum, cobalt,nickel, combinations thereof, or an alloy thereof. The electrodematerial 14 may be deposited over the electrically insulative material12 by conventional techniques such as, for example, electrolessdeposition, PVD, and CVD. The dielectric material 16 may have athickness in a range of from about 10 Å to about 1000 Å and may be anoxide material, a nitride material or a polysilicon material. In oneembodiment, the dielectric material 16 is silicon dioxide. Thedielectric material 16 may be deposited over the electrode material 14by conventional techniques, which are not described in detail herein.

The flowable material 18 may have a thickness in a range of from about100 Å to about 5000 Å and may be deposited over and in contact with thedielectric material 16. As used herein, the term “flowable material”means and includes a material capable of flowing when heated to asufficient temperature. For example, the flowable material 18 may be amaterial that, when heated, forms a viscous liquid or semi-solidmaterial. The flowable material 18 may be a non-reactive, non-conductivematerial and may provide a barrier to moisture. By way of non-limitingexample, the flowable material 18 may be a glass, such as aborophosphosilicate glass (BPSG), a phosphosilicate glass (PSG) or aspin-on glass (SOG). Alternatively, the flowable material 18 may be apolymer or a polyimide. In one embodiment, the flowable material 18 isBPSG. The flowable material 18 may be distributed on the surface of thedielectric material 16 by conventional processes, such as byspin-coating.

The mask material 20 may be a positive or negative photoresist materialor a hardmask material, such as transparent carbon or amorphous carbon.Such photoresist and hardmask materials are known in the art and,therefore, are not described in detail herein. The mask material 20 isillustrated in FIG. 1 as being patterned. The mask material 20 may bedeposited on the flowable material 18, and may be patterned anddeveloped to form an aperture 22 exposing a region of the underlyingflowable material 18. Photoresist materials and photolithographictechniques are well known in the art and, therefore, selecting,depositing, patterning and developing the photoresist material are notdiscussed in detail herein.

FIG. 2 shows the semiconductor structure 10 including a recess 24extending through the flowable material 18, the dielectric material 16and the electrode material 14. Forming the recess 24 produces electrodes14 a and 14 b. The recess 24 is defined by a lower surface 28 of theelectrically insulative material 12 and sidewalls 26 of at least theflowable material 18, the dielectric material 16 and the electrodematerial 14. The recess 24 may have a width in a range of from about 10nm to about 0.1 μm. By way of non-limiting example, the recess 24 may bea two-dimensional structure, such as a container (not shown) having agenerally cylindrical interior, or may be a trench-like structure. Morespecifically, the recess 24 may be a cylindrical capacitor containerstructure. The lower surface 28 of the recess 24 may correspond to anupper surface of the electrically insulative material 12. A mouth 25 ofthe recess 24 may be positioned at an opposite end from the lowersurface 28 of the recess 24. In some embodiments, the electrode material14 may be further removed to form a cavity 17, which is shown in brokenlines. The cavity 17 may be formed using a selective etching processthat removes the electrode material 14 at a faster rate than theelectrically insulative material 12, the dielectric material 16, and theflowable material 18. By way of non-limiting example, if the electrodematerial 14 is tungsten, the electrically insulative material 12 ispolysilicon, the dielectric material 16 is silicon dioxide, and theflowable material 18 is BPSG, a wet etchant includingethylenediaminetetraacetic acid (EDTA) and hydrogen peroxide (H₂O₂) maybe applied at a temperature of about 65° C. to form the cavity 17.

For the sake of clarity, the semiconductor structure 10 depicted in thefollowing drawings includes one recess 24. In other embodiments, aplurality of recesses 24 may be formed in the semiconductor structure10. To form the recess 24, a portion of each of the flowable material18, the dielectric material 16 and the electrode material 14 underlyingthe aperture 22 (shown in FIG. 1) may be removed. Portions of theflowable material 18, the dielectric material 16 and the electrodematerial 14 may be removed using a single etch chemistry, or separateetch chemistries may be used to remove each of these materials. The etchchemistries may be selected by a person of ordinary skill in the artbased on the materials used as the flowable material 18, the dielectricmaterial 16 and the electrode material 14.

As a non-limiting example, if the flowable material 18 is BPSG, a dry(i.e., plasma) etching process using tetrafluoromethane (CF₄) ortrifluoromethane (CHF₃) may be used to remove the portion of theflowable material 18 exposed through the mask material 20, forming themouth 25 of the recess 24. If the dielectric material 16 is silicondioxide, a dry (i.e., plasma) etching process using tetrafluoromethane(CF₄) or trifluoromethane (CHF₃) may be used to remove the exposedportion of the dielectric material 16. If the electrode material 14 isformed from a transition metal, the exposed portion may be removedusing, for example, a carbon monoxide-based plasma etch or a hydrogenchloride and argon plasma etch, to form electrodes 14 a and 14 b.Removing the electrode material 14 may expose at least a portion of theunderlying electrically insulative material 12. As shown by the dashedline in FIG. 2, the recess 24 may, optionally, extend at least partiallyinto the electrically insulative material 12.

While subsequent drawings illustrate the recess 24 as extending at leastpartially into the electrically insulative material 12, the recess 24may terminate at the upper surface of the electrically insulativematerial 12. By way of non-limiting example, if the electricallyinsulative material 12 is silicon dioxide, the exposed portion of theelectrically insulative material 12 may be removed using a buffered HFsolution to extend the recess 24 at least partially into theelectrically insulative material 12. The sidewalls 26 of the recess 24include opposing, exposed vertical regions of the flowable material 18,the dielectric material 16 and the electrodes 14 a and 14 b. The exposedvertical regions of the flowable material 18, the dielectric material 16and the electrodes 14 a and 14 b on opposing sides of the recess 24 maybe contiguous. Alternatively, the sidewalls 26 include exposed verticalregions of the electrically insulative material 12. The exposed verticalregions of the electrically insulative material 12 within the recess 24may oppose one another at contiguous regions within the recess 24. Thelower surface 28 of the recess 24 may be defined by an exposed portionof the electrically insulative material 12.

Referring to FIGS. 3A and 3B, after formation of the recess 24, the maskmaterial 20 may be removed to expose an upper surface 21 of the flowablematerial 18. By way of non-limiting example, if the mask material 20 isa photoresist material, the photoresist material may be removed using aconventional ashing process. Alternatively, the mask material 20 may beremoved after removing a portion of the flowable material 18 whenforming recess 24 such that the remaining portions of the flowablematerial 18 are used as a mask to extend the recess 24 into underlyingdielectric material 16 and the electrode material 14.

As shown in FIG. 3A, a catalyst 30 may be deposited on the sidewalls 26of the recess 24. The catalyst 30 may be a material suitable forcatalyzing the growth of carbon nanotubes. As a non-limiting example,the catalyst 30 may be a transition metal, such as nickel, cobalt, iron,platinum, palladium, copper, vanadium, molybdenum, zinc, an oxide of atransition metal and combinations or alloys thereof. The catalyst 30 maybe deposited on the sidewalls 26 of the recess 24 by electrolessdeposition, spray deposition, photocatalytic deposition, electrochemicaldeposition, chemical vapor deposition, or any other method suitable fordepositing the catalyst 30. In one embodiment, the catalyst 30 is atransition metal capable of being deposited electrolessly. The catalyst30 may be deposited substantially continuously over the lower surface 28and the sidewalls 26 of the recess 24, as shown in FIG. 3A.

Additionally, nanodots (not shown) including a transition metal, such asnickel, cobalt, iron, platinum, palladium, copper, vanadium, molybdenum,zinc, a transition metal oxide, or any combination or alloy thereof, mayused as the catalyst 30. The nanodots may be deposited as adiscontinuous layer having a thickness in a range of from about 2 nm toabout 8 nm, and may include particles having an average diameter in arange of from about 1 nm to about 10 nm. Conventional methods including,but not limited to, plasma enhanced CVD (PECVD) or plasma enhanced ALD(PEALD), may be used to deposit the nanodots on the sidewalls 26 of therecess 24.

The catalyst 30 located on each of the flowable material 18, thedielectric material 16 and the electrically insulative material 12 maybe selectively removed so that the catalyst 30 remains only on sidewalls15 of the electrodes 14 a and 14 b within the recess 24, as shown inFIG. 3B. By way of non-limiting example, if the flowable material 18 isBPSG and both the dielectric material 16 and the electrically insulativematerial 12 are SiO₂, an etch chemistry selective for oxides may be usedto remove the catalyst 30 from the BPSG and the SiO₂. By way ofnon-limiting example, a solution including dilute HF may be used toremove exposed portions of the flowable material 18, the dielectricmaterial 16 and the electrically insulative material 12. By removing theexposed portions of the flowable material 18, the dielectric material 16and the electrically insulative material 12, as shown in broken lines,the catalyst 30 deposited on these materials may be removed withoutremoving the catalyst 30 deposited on the sidewalls 15 of the electrodes14 a and 14 b to produce the semiconductor structure 10 shown in FIG.3B.

In some embodiments, the semiconductor structure 10 includes a cavity 17having surfaces 19 shown in dashed lines of the electrodes 14 a and 14 bexposed within the recess 24. The catalyst 30 may be deposited on thesidewalls 26 of the recess 24 as well as on the surfaces 19 of theelectrode material 14. A protective material (not shown) may be appliedover the sidewalls 26 of the recess 24 and within the cavity 17.Deposition of the catalyst 30 may be followed by a conventional spaceretch to remove the catalyst 30 from the sidewalls 26 of the recess 24and the upper surface 21 of the flowable material 18 without removingthe catalyst 30 from the surfaces 19 of the cavity 17. The protectivematerial may then be removed selective to the catalyst 30 to form thesemiconductor structure 10 shown in FIG. 3B.

The semiconductor structure 10 shown in FIG. 3B may, alternatively, beformed by selectively depositing the catalyst 30 on the sidewalls 15 ofthe electrodes 14 a and 14 b using, for example, an electrolessdeposition process. The catalyst 30 may be selectively deposited on thesidewalls 15 of the electrodes 14 a and 14 b while exposed portions ofthe flowable material 18, the dielectric material 16 and theelectrically insulative material 12 within the recess 24 remainsubstantially free of the catalyst 30.

FIG. 4A shows the semiconductor structure 10 after formation ofnanotubes 32 within the recess 24. As used herein, the term “nanotubes”means and includes any hollow carbon cylinders or graphene cylinders,such as single-walled nanotubes (SWNTs) and multi-walled nanotubes(MWNTs). A plurality of nanotubes 32 may be formed on the catalyst 30within the recess 24 by conventional techniques including, but notlimited to, CVD, arc discharge, and laser vaporization. As anon-limiting example, to initiate formation of the nanotubes 32, thecatalyst 30 on the sidewalls 15 of the electrodes 14 a and 14 b may beexposed to, or contacted with, a process gas. The process gas may be agaseous precursor including a carbon-containing gas or a mixture of thecarbon-containing gas and an inert gas. Non-limiting examples ofcarbon-containing gases include aliphatic hydrocarbons, both saturatedand unsaturated, such as methane, ethane, propane, butane, hexane,ethylene, propylene and combinations thereof; carbon monoxide;oxygenated hydrocarbons, such as acetone, acetylene, methanol andcombinations thereof; and aromatic hydrocarbons, such as toluene,benzene, naphthalene and combinations thereof. In addition, combinationsof the above-mentioned carbon-containing gases may be used. Morespecifically, the carbon-containing gas may be methane, carbon monoxide,acetylene, ethylene or ethanol. Inert gases, such as nitrogen, helium,hydrogen, ammonia or combinations thereof, may be used in the processgas.

The semiconductor structure 10 may be exposed to the process gas in areaction chamber (not shown). The reaction chamber may be, for example,an inductively coupled plasma chamber, a capacitatively coupled plasmachamber, a vacuum chamber, a microwave plasma chamber, or any otherchamber capable of generating high-density plasma. The process gas mayenter the reaction chamber at a flow rate in a range of from about 50cm³/minute to about 2000 cm³/minute, a pressure in a range of from about150 Torr to about 550 Torr and a temperature in a range of from about700° C. to about 1200° C. For example, if cobalt particles are used asthe catalyst 30, nanotubes 32 may be formed by exposing the cobaltparticles to methane gas at a pressure of about 500 Torr and atemperature of about 950° C. As another example, if the catalyst 30includes iron particles, the nanotubes 32 may be formed by exposing theiron particles to methane gas at a pressure of about 200 Torr and atemperature of about 800° C.

Once formation of the nanotubes 32 has been initiated on the catalyst30, the nanotubes 32 self-assemble in the presence of the gaseousprecursor. The nanotubes 32 may have a diameter in a range of from about1 nm to about 10 nm and a length in a range of from about 10 nm to about0.1 μm. The nanotubes 32 may be grown so that a so-called “bridge” isformed between the particles of the catalyst 30 on opposing electrodes14 a and 14 b.

The nanotubes 32 may extend from the particles of catalyst 30 towardsparticles of catalyst 30 on the opposite electrode, providing anelectrical connection between the electrodes 14 a and 14 b. In otherwords, the nanotubes 32 may form an interconnect between the electrodes14 a and 14 b on opposing sides of the recess 24. As used herein, theterm “interconnect” means and includes a structure or structures thatenable communication of an electrical signal between the electrodes 14 aand 14 b. The interconnect may be a path through which data istransmitted, such as an electrically conductive bridge. The interconnectmay be associated with, for example, an interface or connection betweentransistors on a semiconductor die. By growing the nanotubes 32 in situ,electrical contact between the electrodes 14 a and 14 b may besubstantially improved because terminal ends of the nanotubes 32 may beembedded on the surface of, and between, particles of the catalyst 30.In contrast, conventional techniques for forming nanotubes provide onlyphysical contact between the nanotubes and the electrodes. In addition,growing the nanotubes 32 in situ provides improved distribution of thenanotubes 32 at the electrodes 14 a and 14 b than conventionaltechniques of spinning on or spraying the nanotubes.

As shown in FIG. 4B, during formation, the nanotubes 32 may cross overor connect with one another to form at least one junction 34. Nanotubes32 a and 32 b may come into contact with one another at the junction 34.As used herein, the term “contact” means and includes electricalcontact, in which there is a conductive pathway between the nanotubes 32a and 32 b. The crossed nanotubes 32 a and 32 b and the junction 34 mayprovide a memory element able to be switched between at least tworeadable states. Alternatively, a bias may be applied to the nanotubes32 after formation to separate the nanotubes 32 a and 32 b eliminatingthe junction 34. As shown in FIG. 4C, the nanotubes 32 may be formed toextend between surfaces 19 of the electrodes 14 a and 14 b within thecavity 17.

Referring to FIG. 5, the flowable material 18 may be reflowed to form anencapsulating element 38 that covers the mouth 25 of the recess 24(shown in FIG. 2), forming an open volume 40 in the recess 24. Theencapsulating element 38 may span or connect the opposing sidewalls 26of the recess 24. Alternatively, the flowable material 18 may bereflowed such that the encapsulating element 38 connects the opposingsidewalls 26 of the recess 24 and at least partially fills the recess 24while maintaining at least a portion of the open volume 40. Theencapsulating element 38 may be formed by applying sufficient heat toconvert the flowable material 18 to a semi-solid or liquid state. Forexample, if the flowable material 18 is a polymer material, it may beheated to a temperature in a range of from about 150° C. to about 350°C., so that the polymer material flows across the mouth 25 of the recess24 forming the encapsulating element 38. The nanotubes 32 may be stableat a temperature used to reflow the flowable material 18. Sincenanotubes 32 are thermally stable up to a temperature of about 2500° C.in a vacuum, the nanotubes 32 are capable of withstanding the processconditions used to reflow the flowable material 18. As a non-limitingexample, if the flowable material is BPSG, the BPSG may be heated to atemperature of about 600° C. such that the BPSG is present in asemi-solid or liquid state. Once heated, the flowable material 18 maycover the mouth 25 of the recess 24 (shown in FIG. 2) or may flow intoand partially fill the mouth 25 of the recess 24, such as at least aportion of the recess 24 between opposing sidewalls of the flowablematerial 18. However, at least a portion of the recess 24 may remainsubstantially unfilled, maintaining the open volume 40 around thenanotubes 32. As the flowable material 18 cools, the flowable material18 may fuse or consolidate into a substantially continuous material overthe dielectric material 16 and the open volume 40, thus forming theencapsulating element 38. The encapsulating element 38 may form amoisture-resistant barrier that encloses the nanotubes 32 within theopen volume 40 in the semiconductor structure 10. By encapsulating thenanotubes 32 within the open volume 40, the nanotubes 32 may beprotected while maintaining a free space around the nanotubes 32.Additionally, the flowable material 18 may be heated to backfill atleast a portion of the open volume 40, reducing the volume of the openvolume 40 surrounding the nanotubes 32, as shown in FIG. 6.

In another embodiment, a sealing material may be applied over and incontact with the flowable material 18 to cover at least the mouth 25 ofthe recess 24 (shown in FIG. 2), encapsulating the nanotubes 32 andforming the open volume 40. The sealing material may bridge or connectthe opposing sidewalls 26 of the recess 24. By way of non-limitingexample, the sealing material may be a preformed film 44 that may beadhered or otherwise secured to the flowable material 18, as shown inFIG. 7. As a non-limiting example, the preformed film 44 may include adielectric protective material, such as a polyimide or BPSG.Additionally, as shown in FIG. 8, the sealing material, such as thepreformed film 44, may be used in place of the flowable material 18 andmay be applied over and in contact with the dielectric material 16 tocover at least the mouth 25 of the recess 24 (shown in FIG. 2). Thepreformed film 44 may be at least partially fused to the underlyingflowable material 18 or to the underlying dielectric material 16. Thesealing material may also be applied by spin-coating, spray-coating,dip-coating or other conventional techniques. By way of non-limitingexample, the sealing material may be spin-coated or sprayed over theflowable material 18 such that a meniscus (not shown) of the sealingmaterial bridges the mouth 25 of the recess 24, forming the open volume40 and encapsulating the nanotubes 32.

The semiconductor structure 10 may function as an electronic memoryelement in which electrical connection between electrodes 14 a and 14 bis established by the nanotubes 32. By providing an electricalconnection between the electrodes 14 a and 14 b, the nanotubes 32 mayprovide the basis for an electromechanical switching device. Thenanotubes 32 may serve as the memory elements of the electromechanicalswitching device and as elements for switching between a stableconducting (“on”) state and an open (“off”) state. For example, theelectrodes 14 a and 14 b may be source and drain electrodes and thenanotubes 32 may be suspended therebetween. The electrodes 14 a and 14 bmay be operably coupled to a voltage source and ground so that a currentis passed between the electrodes 14 a and 14 b by the nanotubes 32.Switching between the on and off states may be performed by applying avoltage across the nanotubes 32. The voltage may be applied to thenanotubes 32, for example, through the electrodes 14 a and 14 b. By wayof non-limiting example, a voltage in a range of from about 0.5 Volts toabout 5 Volts may be applied between the electrodes 14 a and 14 b. Whena sufficient voltage is applied, the semiconductor structure 10 remainsin the on state. The semiconductor structure 10 may be switched to theoff state by a change in the voltage applied. For example, the crossednanotubes 32 a and 32 b which intersect at the junction 34 may be cut ordisconnected by applying a sufficient bias to the nanotubes 32. Thenanotubes 32 may be switched between on and off states by alternativelybiasing the nanotubes 32 or by biasing the nanotubes 32 at the junction34. The application of a current may induce the nanotubes 32 a and 32 bto deform and connect such that the electromechanical switching deviceis in the on state. When the current flow is restricted, the nanotubes32 a and 32 b disconnect such that the electromechanical switchingdevice is in the off state.

FIG. 9 shows another embodiment of a semiconductor structure 100 thatincludes a recess 108 formed through an electrode material 104 and adielectric material 106 overlying an electrically insulative material102. The electrode material 104 may be formed over and in contact withthe electrically insulative material 102; the dielectric material 106may be formed over and in contact with the electrode material 104. Theelectrically insulative material 102, the electrode material 104, thedielectric material 106 may be formed by conventional techniques, whichare not described in detail herein.

For example, the electrically insulative material 102 may be formed froma dielectric material such as silicon dioxide (“SiO₂”). In someembodiments, the electrically insulative material 102 may be depositedover a substrate (not shown) by conventional techniques, which are notdescribed in detail herein. By way of non-limiting example, theelectrode material 104 may be formed from a semiconductive material or aconductive material, such as doped polysilicon material. The electrodematerial 104 may be deposited over the electrically insulative material102 by conventional techniques such as, for example, by CVD. Thedielectric material 106 may be an oxide material, a nitride material ora polysilicon material. In one embodiment, the dielectric material 106is silicon dioxide. The dielectric material 106 may be deposited overthe electrode material 104 by conventional techniques, which are notdescribed in detail herein.

As shown in FIG. 9, the recess 108 is defined by a surface 110 of theelectrically insulative material 102 and sidewalls 112 includingopposing, exposed vertical regions of the dielectric material 106 andthe electrode material 104. For the sake of clarity, the semiconductorstructure 100 depicted in the following drawings includes one recess108. In other embodiments, a plurality of recesses (not shown) may beformed in the semiconductor structure 100. To form the recess 108, aportion of each of the dielectric material 106 and the electrodematerial 104 may be removed using conventional techniques. For example,portions of the dielectric material 106 and the electrode material 104may be removed selective to a mask material (not shown) using a singleetch chemistry, or separate etch chemistries. The etch chemistries maybe selected by a person of ordinary skill in the art based on thematerials used as the dielectric material 106 and the electrode material104. By way of non-limiting example, if the dielectric material 106 issilicon dioxide, a suitable dry (i.e., plasma) etching process using,for example, tetrafluoromethane or trifluoromethane, may be employed toremove a portion of the dielectric material 106 selective to the maskmaterial. The electrode material 104 may be selectively removed usingthe underlying electrically insulative material 102 as an etch stop. Ifthe electrode material 104 is a doped polysilicon and the electricallyinsulative material 102 is silicon dioxide, a hot phosphoric acid etchmay be used to remove a portion of the electrode material 104 withoutsubstantially removing the underlying electrically insulative material102.

As shown in FIG. 10, a catalyst 114 may be deposited on the exposedvertical regions of the electrode material 104 and at least one nanotubemay be formed thereon using conventional techniques, such as thosedescribed above. Conventional techniques of forming carbon nanotubes mayresult in the formation of a mixture of carbon nanotubes withmetallic-type properties, which are referred to herein as “metallicnanotubes,” and carbon nanotubes with semiconducting-type properties,which are referred to herein as “semiconducting nanotubes.” The metallicnanotubes may be removed or damaged using conventional techniques, suchas selective etching using a hydrogen or methane plasma, electricalbreakdown or microwave heating. By way of non-limiting example, asufficient current may be passed through the nanotubes to selectivelydamage or destroy the metallic nanotubes while preserving thesemiconducting nanotubes.

After substantially removing the metallic nanotubes, at least onesemiconducting nanotube 116 may remain. For the sake of clarity, thesemiconductor structure 100 depicted in the following drawings includesa single semiconducting nanotube 116. In other embodiments, a pluralityof semiconducting nanotubes 116 may be formed in the semiconductorstructure 100. The remaining semiconducting nanotubes 116 mayelectrically couple the exposed regions of the electrode material 104.The electrode material 104 may function as source and drain regions 104a and 104 b of the semiconductor structure 100.

Referring to FIG. 11, a gate dielectric 118 may be conformally formedover and in contact with the dielectric material 106, the sidewalls 112of the dielectric material 106 and the source and drain regions 104 aand 104 b, and the surface 110 of the electrically insulative material102. The gate dielectric 118 may be deposited over or may encase orsurround the semiconducting nanotubes 116. By way of non-limitingexample, the gate dielectric 118 may be silicon dioxide or a high-kmaterial. Conventional methods including, but not limited to, PVD andCVD, may be used to deposit the gate dielectric 118.

As depicted in FIG. 12, a conductive material 120 may be conformallyformed over and in contact with the gate dielectric 118. The conductivematerial 120 may be a metal such as tungsten, titanium, tantalum,aluminum, platinum, gold, silver, copper, or combinations thereof. Theconductive material 120 may be deposited over the gate dielectric 118and dielectric material 106 by conventional techniques such as, forexample, electroless deposition, PVD, and CVD. The conductive material120 is isolated from the semiconducting nanotubes 116 by the gatedielectric 118.

FIG. 13 shows the semiconductor structure 100 after removal of a portionof the conductive material 120, the gate dielectric 118, and thedielectric material 106 to form a gate 122. The materials may be removedto render an upper surface 124 of the semiconductor structure 100substantially planar. For example, conventional planarization techniquesmay be used to form the gate 122 including the gate dielectric 118disposed over the semiconducting nanotubes 116 and lining the remainingportions of the sidewalls 112 and the conductive material 120 disposedover the gate dielectric 118. The semiconducting nanotubes 116 remainelectrically connected to the source and drain regions 104 a and 104 band may function as a current carrying device or channel.

The semiconductor structure 100 may function as a control gate in whichthe semiconducting nanotubes 116 function as a semiconducting channelbetween the source and drain regions 104 a and 104 b. The semiconductingchannel may include a plurality of semiconducting nanotubes 116 or,alternatively, a single nanotube. By applying a current to the gate 122,the semiconducting nanotubes 116 can be switched from a conducting to aninsulating state. When an appropriate voltage is applied to the gate122, the semiconducting nanotubes 116 may function as a channel thatpasses a current between the source and drain regions 104 a and 104 b.

While the invention may be susceptible to various modifications andalternative forms, specific embodiments have been shown by way ofexample in the drawings and have been described in detail herein.However, it should be understood that the invention is not intended tobe limited to the particular forms disclosed. Rather, the inventionencompasses all modifications, variations and alternatives fallingwithin the scope of the invention as defined by the following appendedclaims and their legal equivalents.

1. A method for forming a semiconductor device, comprising: forming asemiconductor structure comprising a flowable material, a dielectricmaterial, an electrode material, and an electrically insulativematerial; removing a portion of each of the flowable material, thedielectric material, and the electrode material from the semiconductorstructure to form at least one recess therein; forming a catalystmaterial on opposing sidewalls of the electrode material within the atleast one recess; forming a plurality of carbon nanotubes extending fromparticles of the catalyst material on one of the opposing sidewalls ofthe electrode material to particles of the catalyst material on anotherof the opposing sidewalls of the electrode material; and sealing a mouthof the at least one recess.
 2. The method of claim 1, further comprisingremoving a portion of the electrically insulative material exposed at abottom surface of the at least one recess.
 3. The method of claim 1,further comprising applying a current through the plurality of carbonnanotubes to separate at least one junction comprising interconnectedcarbon nanotubes.
 4. The method of claim 1, wherein removing the portionof each of the flowable material, the dielectric material, and theelectrode material from the semiconductor structure to form the at leastone recess therein comprises forming the at least one recess defined bycontiguous, opposing sidewalls of the flowable material, the dielectricmaterial, and the electrode material.
 5. The method of claim 1, whereinforming the plurality of carbon nanotubes extending from the particlesof the catalyst material on the one of the opposing sidewalls of theelectrode material to the particles of the catalyst material on theanother of the opposing sidewalls of the electrode material comprisesforming the plurality of carbon nanotubes in electrical contact with theelectrode material on opposing sides of the at least one recess.
 6. Themethod of claim 1, wherein forming the plurality of carbon nanotubesextending from the particles of the catalyst material on the one of theopposing sidewalls of the electrode material to the particles of thecatalyst material on the another of the opposing sidewalls of theelectrode material comprises forming the plurality of carbon nanotubesextending between metal catalyst particles on the one of the opposingsidewalls of the electrode material and the another of the opposingsidewalls of the electrode material by contacting the metal catalystparticles with methane gas at a temperature of 800° C. and a pressure ofabout 200 Torr.
 7. The method of claim 1, wherein forming the catalystmaterial on opposing sidewalls of the electrode material within the atleast one recess comprises selectively depositing the catalyst materialon the opposing sidewalls of the electrode material within the at leastone recess.
 8. The method of claim 1, wherein forming the catalystmaterial on opposing sidewalls of the electrode material comprises:depositing the catalyst material on exposed portions of the flowablematerial, the dielectric material, the electrode material, and theelectrically insulative material; and removing a portion of each of theflowable material, the dielectric material, and the electricallyinsulative material and the catalyst material thereon withoutsubstantially removing the electrode material and the catalyst materialthereon.
 9. The method of claim 1, wherein sealing the mouth of the atleast one recess comprises applying a dielectric protective materialover the flowable material and leaving an open volume within the atleast one recess, the open volume defined by the opposing sidewalls ofeach of the electrode material and the dielectric material, a surface ofthe electrically insulative material, and a surface of the dielectricprotective material.
 10. The method of claim 1, wherein sealing themouth of the at least one recess comprises heating the flowable materialto at least cover the mouth of the at least one recess with the flowablematerial.
 11. The method of claim 1, wherein sealing a mouth of the atleast one recess comprises applying a preformed film over and in contactwith the flowable material to at least cover the mouth of the at leastone recess and to form an open volume surrounding the plurality ofcarbon nanotubes.
 12. A semiconductor structure, comprising: at leastone open volume defined by a lower surface of an encapsulating element,an upper surface of an electrically insulative material, and contiguoussidewalls of each of a dielectric material and opposing electrodes; acatalyst material disposed on the sidewalls of the opposing electrodes;and a plurality of carbon nanotubes electrically connecting the opposingelectrodes within the at least one open volume and extending fromparticles of the catalyst material on a sidewall of one of the opposingelectrodes to particles of the catalyst material on a sidewall ofanother of the opposing electrodes.
 13. The semiconductor structure ofclaim 12, wherein the encapsulating element comprises a flowablematerial.
 14. The semiconductor structure of claim 12, wherein theencapsulating element comprises a preformed film disposed over an uppersurface of the open volume.
 15. The semiconductor structure of claim 12,wherein opposite ends of each of the plurality of carbon nanotubes arein physical contact with the particles of the catalyst material disposedon the sidewalls of the opposing electrodes.
 16. The semiconductorstructure of claim 12, wherein at least one carbon nanotube of theplurality of carbon nanotubes is in contact with at least another carbonnanotube of the plurality of carbon nanotubes.
 17. A method for forminga semiconductor device, comprising: forming a semiconductor structurecomprising a flowable material, a dielectric material, an electrodematerial, and an electrically insulative material; removing a portion ofeach of the flowable material, the dielectric material, and theelectrode material from the semiconductor structure to form at least onerecess therein; forming a plurality of carbon nanotubes extendingbetween a catalyst on opposing sidewalls of the electrode materialwithin the at least one recess, comprising: selectively depositing thecatalyst on the opposing sidewalls of the electrode material within theat least one recess; and exposing the catalyst to a carbon-containinggas to form the plurality of carbon nanotubes extending from particlesof the catalyst on one of the opposing sidewalls of the electrodematerial to particles of the catalyst on another of the opposingsidewalls of the electrode material; and sealing a mouth of the at leastone recess.
 18. The method of claim 1, wherein forming the catalystmaterial on the opposing sidewalls of the electrode material within theat least one recess comprises forming the catalyst materialsubstantially continuously over a lower surface and sidewalls of the atleast one recess.
 19. The method of claim 18, further comprisingselectively removing the catalyst material from the lower surface andthe sidewalls of the at least one recess such that the catalyst materialremains only on the opposing sidewalls of the electrode material.